This invention provides a pipe source of ultraviolet radiation which uses cathode arrays to emit electron beam current to excite cathodoluminescent phosphors emitting in the ultraviolet (UV) portion of the electromagnetic spectrum (100 to 400 nanometers in wavelength). The phosphors can be selected to emit in any part or parts of the UV bands. UV-A and UV-B phosphors can be used for in various industrial applications. UV-C phosphors can be incorporated in the pipe sources of this invention for applications such as water or air purification, through either direct or photocatalytic sterilization of contaminants. Fluids such as air or water flow through the pipe and are evenly irradiated with UV flux. In certain aspects of this invention, the ultraviolet phosphors can be mixed together to provide a desired multi-spectral output. In other aspects, different wavelength phosphors can be deposited on different parts of the phosphor pipe, so that the different spectra can be selectively addressed for light emission.
Most UV sources now used are fluorescent gas discharge tubes or lamps, most commonly with a low or medium pressure mercury vapor medium for the gas discharge. These sources have a number of limitations, including the hazard of the mercury in the tubes, risks of breakage, narrow spectral range, low power efficiency, especially in the case of medium pressure mercury vapor tubes, sensitivity to temperature variations, heat generation, and difficulties in cleaning and maintenance in some applications. UV light emitting diodes (LEDs) have been developed more recently. These have low power efficiency below about 365 nm in wavelength and also suffer from “droop”, a phenomenon in which power efficiency drops further as power output is increased. LEDs are also made on compound semiconductor wafers such as AlGaN, so they are expensive to begin with and then have to be diced and assembled for larger area applications, which adds further to the cost of a wide area UV source.
U.S. Pat. Nos. 4,274,028 and 7,300,634 disclose flat panel sources of cathodoluminescent UV flux in which the phosphors are excited by electron beam current emitted from cold cathode films or cold cathode arrays. For sterilization or other treatment of fluids such as air or water, a pipe source of UV flux would provide increased efficiency, safety and convenience.